One form of valve in common use in subsea applications is a rotary gate valve. Such a valve has a valve member engageable with a seat to control fluid flow between a pair of ports thereof. The valve may, if desired, control fluid flow in both directions between the ports. The valve member is angularly movable between a closed position in which fluid flow between the ports is not permitted, and an open position in which such flow may take place. Typically, the valve member is moved through an angle of approximately 90° in moving between the open and closed positions.
It is often desirable to be able to drive the valve between its open and closed positions from a remote location. One form of actuator used to drive such a valve for movement is a hydraulically operated actuator. A known form of hydraulic actuator comprises a piston moveable under the application of fluid under high pressure thereto between first and second positions. The piston carries one or more pins which extend into angled slots formed in a drive member such that axial movement of the piston causes angular movement of the drive member. By connection of the drive member to the valve member of a valve, it will be appreciated that adjustment of the position of the valve can be achieved through appropriate control of the fluid pressures applied to the piston.
The piston is typically spring biased towards a known position, for example it may be biased towards a valve closed position so that, in the absence of the application of pressure thereto, the valve will be closed, the valve only occupying its open position when hydraulic fluid is applied to the piston, the valve thus taking the form of a fail safe closed or FSC valve.
In the event of a failure within the control system or actuator, it may be necessary to manually adjust the valve position. Where the valve is located in a subsea location, this will typically be undertaken by a diver or ROV. In one known form of actuator, such manual adjustment first requires the diver or ROV to apply an axial load to a manual override actuator to disengage a clutch provided between the drive member and the valve member, and for the diver or ROV to then rotate the manual override actuator to drive the valve member to the desired position. The application of the axial load is an undesirable complication for a diver or ROV to achieve. Accordingly, such manual override is undesirable. Furthermore, there is insufficient visual indication of whether or not the manual override is engaged.